1. Field of the Invention
The present invention relates to layer 2 and layer 3 switching of data packets in a non-blocking network switch configured for switching data packets between subnetworks.
2. Background Art
Local area networks use a network cable or other media to link stations on the network. Each local area network architecture uses a media access control (MAC) enabling network interface devices at each network node to access the network medium.
The Ethernet protocol IEEE 802.3 has evolved to specify a half-duplex media access mechanism and a full-duplex media access mechanism for transmission of data packets. The full-duplex media access mechanism provides a two-way, point-to-point communication link between two network elements, for example between a network node and a switched hub.
Switched local area networks are encountering increasing demands for higher speed connectivity, more flexible switching performance, and the ability to accommodate more complex network architectures. For example, commonly-assigned U.S. Pat. No. 5,953,335 discloses a network switch configured for switching layer 2 type Ethernet (IEEE 802.3) data packets between different network nodes; a received data packet may include a VLAN (virtual LAN) tagged frame according to IEEE 802.1q protocol that specifies another subnetwork (via a router) or a prescribed group of stations. Since the switching occurs at the layer 2 level, a router is typically necessary to transfer the data packet between subnetworks. A router is distinguishable from an integrated network switch in that the router transfers packets by executing software routines, whereas an integrated network switch is an integrated silicon chip configured for switching data packets based on control data and switching data stored on-chip.
Current layer 2 switches preferably are configured for operating in a non-blocking mode, where data packets can be output from the switch at the same rate that the data packets are received. Newer designs are needed to ensure that higher speed switches can provide both layer 2 switching and layer 3 switching capabilities for faster speed networks such as 100 Mbps or gigabit networks.
However, it becomes increasingly difficult for the switching fabric of a network switch to be able to perform layer 3 processing at the wire rates (i.e., the network data rate). For example, conventional network switches use a centralized switching core having a centralized address lookup table that stores addressing information and frame forwarding decisions for the corresponding address entries, for example 4096 (4k) entries. Layer 3 processing, however, requires implementation of user-defined policies that include searching a large number of fields for specific values. These user-defined policies may specify what type of data traffic may be given priority accesses at prescribed intervals; for example, one user defined policy may limit Internet browsing by employees during work hours, and another user-defined policy may assign a high priority to e-mail messages from corporate executives.
In addition, different layer 3 protocols (e.g., Internet Protocol, IPX, DECnet, etc.) may be encountered by a centralized device such as a router. Hence, layer 3 processing may require the centralized device to identify the layer 3 protocol in use by the received data packet, and then perform the layer 3 processing according to the identified layer 3 protocol.
Hence, the number of such user policies may be very large, posing a substantial burden on performance of layer 3 processing at the wire rates, especially if the centralized switching core needs to rely on accessing a centralized address table for the multiple network switch ports. Hence, implementation of a large number of user policies may require a substantially larger number of address entries, substantially increasing the size of the centralized address lookup table to an unreasonable size that renders implementation in silicon for wire-rate switching unfeasable.